Grain Drying (cont.)
Energy,
Quality, Fire, Moisture and Fans
 Energy Efficiency
Selecting a Drying
System
Maintaining
Quality During Drying
Drying Fire Hazard
Moisture
Determination
Moisture Shrink
Selecting Fans
Introduction
Drying Advantages and Disadvantages
Recommended
Storage Moisture Contents and
Estimated
Allowable Storage times
Influence
of Drying Conditions
Types of Dryers and
Drying
Natural Air/Low
Temperature Drying
Layer
Drying
High
Temperature Bin Drying
Column
Dryers
Combination
Drying
Dryeration
and In Storage Cooling
Heaters,
Costs, Safety and Managing Stored Grain
Selecting a
Heater
Drying
System Cost
Grain
Handling Systems
Safety
Considerations
Managing
Stored Grain
Other
Drying and Storage Information Available
Dryer design is a compromise between speed, energy
efficiency and moisture variation across the grain dryer
column.
Figure 11 shows how airflow and temperature affect the
moisture variation across a grain column one foot thick
when drying corn from 25 percent to 15 percent moisture.
Figure 11. Predicted
moisture content differentials of corn leaving a
conventional crossflow dryer as a function of drying
air temperature and airflow rate. Column width is 12
inches. (University of Nebraska, Dr.
Thompson)
(7KB b&w diagram)
The least moisture variation was obtained at low
temperatures and high airflow rates. However, Figure 12
shows that these conditions result in high energy
requirements. For energy efficiency, a high temperature
and low airflow rate are required. Each grain reacts
differently to high temperatures; low temperatures may be
needed to maintain grain quality.
Figure 12.
Energy requirements of a conventional crossflow dryer
as a function of drying air temperature and airflow
rate. (University of Nebraska)
(8KB b&w diagram)
Table 5 shows the estimated drying energy requirement
of some dryer types. Generally more energy is required
the faster the grain is dried.
Table 5. Estimated Drying Energy
Requirements for Some Dryer Types.
---------------------------------------
BTU's/lb. of
Dryer Type Water Removed
---------------------------------------
Natural Air 1000 - 1200
Low Temperature 1200 - 1500
Batch-in-Bin 1500 - 2000
High Temperature
Air Recirculating 1800 - 2200
W/0 Air Recirculating 2000 - 3000
---------------------------------------
Other factors affecting the energy required for drying
grain include the moisture content at harvest and
moisture content for storage. If the grain dries in the
field, no fuel is required for drying but field losses,
both grain quantity and quality, may be higher. Avoiding
overdrying also results in energy savings since less
water is removed.
The energy savings to be obtained from a grain dryer
with air-recirculation capability depends on the type and
the amount of air recirculated in the dryer and the
difference between the recirculation air temperatures and
the drying temperature. Dryers that can recirculate only
the cooling air show an energy efficiency increase of 10
to 20 percent compared to equivalent dryers without air
recirculation. Units capable of recirculating both the
cooling and part of the drying air may improve energy
efficiency up to 30 percent. Air recirculation may
increase the fire hazard when drying sunflower, so it is
not normally recommended.
For more information on energy conservation, refer to
National Corn Handbook leaflet NCH-14, "Energy
Conservation and Alternative Energy Sources for Corn
Drying."
Each drying system has advantages and limitations.
Select a dryer and system to meet your particular need.
It is important to weigh all factors in the drying
system before selecting a dryer. For example, a
continuous flow column dryer needs holding facilities for
both wet and dry grain. Grain handling equipment is a
very important part of any drying system. Large electric
motors that need adequate electrical service are likely
involved. Fuel must be stored and supplied to the dryer.
The drying system needs to be located to allow for good
traffic flow and drainage. Existing storage facilities
and handling equipment also need to be considered.
Purchase the dryer or equipment from a reputable
dealer that will be able to service the dryer and provide
other assistance. Check with people who have dealt with
the dealer you are considering. Examine existing systems
or dryer installations and visit with the operators about
their experiences.
Planning for the future primarily means leaving space
for future facilities and equipment. One rule of thumb is
to plan for the foreseeable future and then double it.
This allows an orderly development as the system expands.
Structures in place cannot be easily changed.
Nutritional quality of livestock feed is unaffected by
temperatures up to 250F, although some kernel surface
scorching may result.
A number of tests on drying seed grains show that
germination drops rapidly as the seed kernel temperature
goes above 120F. For this reason it is recommended that a
maximum of 110F air be used. Some seed producers use
lower temperature limits in an effort to provide some
extra protection. Since germination is important for
malting barley, 110 to 120F is the maximum recommended
drying air temperature.
For flour milling, it is important that temperatures
above about 150F be avoided because of the effect of high
temperatures on the chemical structure of the grain. It
is a common practice for some millers to test a sample of
the grain for milling properties before purchasing. High
temperatures can severely damage baking quality even
though the grain kernels appear undamaged. The maximum
drying air temperature for drying milling wheat is 150F
for 16 percent moisture content and 130F for 20 percent
moisture content wheat.
Rate of drying, which is related to drying
temperature, is the major limitation on drying beans. At
high drying rates, the seed coat of soybeans shrinks
faster than the seed, causing cracks in the seed coat.
Further handling results in breaking and removal of the
seed coat. Development of bitter or "off"
flavors and increased spoilage can occur in the split
seed. If the relative humidity of the drying air is kept
above 40 percent, there is little or no damage to the
seed coat. Pinto beans may develop stress cracks and
splits anytime the relative humidity of the drying air
drops below 40 percent. A 10F temperature rise will
reduce the relative humidity by about one-fourth. If air
is 40F and 80 percent relative humidity, heating it 10F
will reduce the relative humidity to about 60 percent;
80–(80�4). Natural air/low temperature drying is
best for drying beans.
Oil yield and fatty acid composition of sunflower are
not affected by drying air temperatures up to 220F.
Non-oil sunflower seed meats may be scorched at
temperatures exceeding 180 to 190F.
Maximum recommended drying air temperatures for
selected grains are shown in table 6.
Table 6. Maximum Recommended Drying Air
Temperatures for Selected Grains. (�F)
--------------------------------------------------------
Dryer Type Seed
-----------------------------------
Cont. Recirculating Column Bin
Grain Flow Batch Batch Batch
Dryer Dryer Dryer Dryer
--------------------------------------------------------
Wheat and 150� 150� 135� 120� 110�
Durum
Malting 120� 120� 110� 110� 110�
Barley
Soybeans 130� 130� 110� 110� 110�
(non-food)
Oats 150� 150� 135� 120� 110�
Rye 150� 150� 135� 120� 110�
Sunflower 200� 200� 180� 120� 110�
Flaxseed 180� 180� 160� 120� 110�
Corn 200� 200� 180� 120� 110�
Mustard and 150� 150� 130� 110� 110�
Rape
Pinto Beans, 90� 90� 90� 90� 90�
Navy Beans
--------------------------------------------------------
Any dryer using an open flame to heat the air poses a
constant fire hazard when used to dry any crop,
especially sunflower and sorghum. Fine fibers from
sunflower seed or other plant materials may be ignited by
the burner and carried to the seeds, causing them to
ignite. This fire hazard can be reduced by turning
portable dryers into the wind so airborne fibers are
blown away from the dryer intake and by pointing
permanent dryers into the prevailing wind. A moveable air
intake duct may be placed on the burner intake to draw
clean air away from the dryer. The duct must be large
enough to not restrict the airflow, because drying speed
will be reduced if the airflow is reduced.
Clean the dryer, air ducts, and area around the dryer
at least daily. Frequently remove the collection of
sunflower lint on the dryer column and in the plenum
chamber, as this material becomes extremely dry and can
be ignited during dryer operation. A major concern is
that some sunflower seeds will hang up in the dryer or be
stopped by an accumulation of fines and become over
dried. Make sure the dryer is completely cleaned out
after each batch, keep sunflower seed moving in the
dryer, and check a continuous flow dryer regularly
(hourly) to see that the sunflower seed are moving.
High speed dryers are like a forge when a fire gets
going. However, fires can be controlled if they are
noticed immediately, which makes constant monitoring
necessary. Many fires can be extinguished by just
shutting off the fan to cut off the oxygen. A little
water applied directly to the fire at the early stages
may extinguish it if shutting off the fan fails to do so.
A fire extinguisher for oil type fires should be used for
oil sunflower fires. Many dryers are now designed so that
sunflower can be unloaded rapidly in case of a fire,
before the dryer is damaged. In some dryers, just the
part of the dryer affected by the fire needs to be
unloaded.
Grain moisture content may be determined by direct or
indirect methods. Direct methods are commonly used for
laboratory work where exact determination is critical.
Heating the grain sample to drive off moisture and
weighing before and after heating, according to a
standardized procedure, to find water loss is a direct
method.
Moisture meters commonly used with farm drying
installations measure moisture indirectly. They measure
the electrical conductance or capacitance of the grain,
since moisture in grain affects these electrical
properties of the kernels. A reading on the moisture
meter is converted to a moisture reading by use of a
calibration chart or table.
Most farm moisture meters have accuracies of � �
percent moisture content under normal operating
conditions. High grain temperatures affect the accuracy
of moisture meter readings. Grain close to the meter's
calibration temperature, often about 75F, give more
accurate readings than grain at higher or lower
temperatures. Some meters have an internal temperature
compensation and others require that the temperatures be
measured and a correction be made to the meter reading
using a correction chart.
Many stories of moisture contents
"rebounding" after drying actually are caused
by grain tempering. Grain that has just been dried will
have an uneven moisture content across the kernel. The
kernel surface will be drier than the interior, and this
will cause the moisture meter to read low. During
tempering, the moisture redistributes in the kernel,
which gives a more accurate but higher moisture reading.
When checking samples for moisture directly from the
dryer, a correction factor may have to be added. The
factor changes with temperature and moisture content, so
the factor must be determined periodically for your
meter. One method of approximating this correction is to
seal a sample in a canning jar for 10 to 12 hours until
the temperatures and moisture distribution within the
kernels equalize. Then check the moisture content.
Comparing this reading to the moisture readings of the
sample straight from the dryer will give an approximate
correction factor.
Caution: Recheck grain 12 hours after drying to
be sure the moisture content is what you want.
Points to remember when using moisture testers are:
- When testing during or immediately after drying,
the reading is probably in error.
- Find the moisture content of several samples for
the lot of grain being checked.
- Do not handle the sample with your hands (this
adds moisture) or expose it to air in an open
container (this causes some drying or wetting to
occur)
- Weigh or measure the sample accurately if
required.
- Use proper procedure for temperature correction
if necessary.
The removal of moisture from grain during drying
causes a reduction in grain quantity referred to as
moisture shrink. The moisture shrink can be calculated
using the following equation.
Moisture Shrink (%) =
Initial Moisture Content -
Final Moisture Content
----------------------------- x 100
100 - Final Moisture Content
The moisture shrink percentage for drying corn from 25
to 15 percent moisture content is:
Moisture Shrink (%) =
25 - 15
-------- x 100 = 11.76%
100 - 15
The weight reduction drying 1000 pounds of corn from
25 to 15 percent moisture content is 11.76% x 1000 =
117.6 pounds. Moisture shrink tables AE-94, "Grain
Drying Tables," are available from the NDSU
Extension Service.
Refer to NDSU Extension Circular AE-905, "Grain
Moisture Content Effects and Management," for more
information on moisture shrink and other effects of
changing grain moisture content.
Selecting Fans
Satisfactory drying depends upon both the airflow rate
supplied and the ability of the air to hold water. The
ability of a fan to move air through the grain will
depend upon the fan design, and the resistance to the
airflow. The pressure a fan must develop to overcome the
resistance of grain to airflow is referred to as static
pressure. The unit most commonly used for measuring
resistance to airflow is inches of water as measured by a
manometer (Figure 13). One inch of water is equal to
0.036 pounds per square inch (psi). Figure 14 shows
typical resistances to airflow of some clean grains
commonly grown in North Dakota. This resistance to
airflow is technically referred to as static pressure
drop through the grain. Multiply this pressure drop for
clean grain by 1.3 to 1.5 to adjust for packing and
foreign material in the grain. The value varies depending
on the cleanliness and physical properties of the grain.
A value of 1.3 is commonly used for wheat and 1.5 for
other crops. Tables 7 and 8 list the estimated static
pressure for various airflow rates and grain depths for
bin drying clean grain. Refer to NDSU Extension Bulletin
EB-35, "Natural Air/Low Temperature Crop
Drying," for more detailed low airflow rate tables.
Figure 13.
A U-tube manometer used for measuring static
pressure.
(15KB b&w diagram)
Figure 14.
Resistance of clean grains to air flow. Multiply
values by 1.3 to 1.5 to adjust for packing and
foreign material. Using a grain distributor will
increase the resistance to airflow.
(24KB b&w diagram)
Table 7. Estimated Static Pressures for
Various Airflow Rates and Grain Depths
for Bin Drying Clean Grain. Increase Values
Slightly to Account for Foreign Material.
--------------------------------------------------
Airflow Rate
--------------------------------------
Depth of 8 cfm/bu 6 cfm/bu 5 cfm/bu 3 cfm/bu
Grain --------------------------------------
Static pressure inches of water
--------------------------------------------------
Wheat
4 feet 3.25 2.25 1.85 1.13
6 feet 7.45 5.35 3.79 2.35
8 feet 16.25 10.65 8.25 4.25
10 feet 27.75 18.25 13.25 7.75
Barley
4 feet 2.10 1.45 1.25 .77
6 feet 5.05 3.37 2.65 1.45
8 feet 12.25 6.65 5.05 2.65
10 feet 18.25 12.25 8.75 4.25
Soybeans
4 feet 1.00 .77 .61 .44
6 feet 2.35 1.57 1.27 .76
8 feet 4.65 3.30 2.40 1.21
10 feet 7.75 5.15 4.05 1.95
12 feet 12.25 7.69 4.81 2.89
Shelled
Corn
4 feet 1.36 .93 .81 .51
6 feet 2.95 2.11 1.69 1.00
8 feet 7.05 4.25 3.45 1.61
10 feet 13.25 7.25 5.45 2.75
12 feet 21.85 11.17 8.65 4.33
--------------------------------------------------
Use barley values for sunflower.
Table 8. Estimated Static Pressures For
Various Airflow Rates and Grain Depths
for Natural Air/Low Temperature Bin Drying.*
----------------------------------------------
Airflow Rate (cfm/bu.)
-----------------------
Grain Depth (ft.) 1/2 3/4 1 2
----------------------------------------------
inches of water column
Wheat 10.0 1.5 2.1 2.8 5.7
12.5 2.2 3.2 4.2 9.1
15.0 2.9 4.4 5.7 13.8
17.5 3.9 5.9 8.4 20.0
20.0 5.1 7.7 10.6 24.7
----------------------------------------------
Barley 10.0 1.2 1.6 2.0 4.0
Oats 12.5 1.6 2.2 3.0 6.5
Sunflower 15.0 2.1 3.2 4.1 9.6
17.5 2.7 4.1 5.8 14.2
20.0 3.5 5.3 7.4 18.5
----------------------------------------------
Shelled 10.0 0.8 1.0 1.3 2.5
Corn 12.5 1.0 1.4 1.8 3.9
15.0 1.3 1.9 2.2 5.9
17.5 1.6 2.3 3.7 8.6
20.0 2.1 3.1 4.4 11.6
----------------------------------------------
Soybeans 10.0 0.7 0.9 1.0 1.5
12.5 1.0 1.2 1.6 2.1
15.0 1.1 1.4 1.8 3.1
17.5 1.2 1.6 2.0 4.3
20.0 1.5 2.2 2.9 5.9
----------------------------------------------
* Includes 0.5 inch of static pressure drop
for the distribution system.
Table 9 lists estimated static pressures for various
airflow rates and column widths typically used for column
dryers.
Table 9. Estimated Static Pressures for
Various Airflow Rates and Thicknesses of
Grain for Batch and Continuous Flow Dryers.
----------------------------------------------
Air Flow Rate (cfm/bu)
Depth of ------------------------------------
Grain 25 50 75 100 150
----------------------------------------------
(Wheat)
8" 0.21 0.53 0.87 1.30 2.70
12" 0.54 1.30 2.20 3.10 6.80
16" 1.00 2.50 4.10 6.10 12.60
20" 1.80 4.20 6.80 10.00 21.70
24" 2.60 6.20 10.40 15.20
----------------------------------------------
(Corn)
8" 0.07 0.18 0.35 0.54 1.60
12" 0.18 0.53 1.10 1.70 4.70
16" 0.37 1.10 2.30 3.60 10.10
20" 0.68 2.20 4.00 6.20 17.50
24" 1.10 3.40 6.60 10.00
----------------------------------------------
* Note: This table gives values for clean, dry,
unpacked grain. Under actual drying conditions
higher values may be expected.
There are several different types of fans. Each has
specific operating characteristics and applications. The
common types of fans used for grain drying applications
are the axial-flow, low speed centrifugal, high speed
centrifugal, and the in-line centrifugal (Figure 15).
Figure 15.
Common fans used on grain systems.
(13KB b&w diagram)
Table 10 shows the airflow at various static pressures
for some typical fans. NOTE: A similar table for the
specific fan being considered should be consulted when
selecting a fan.
Fan Selection Example: Select a fan to
provide an airflow rate of 1.0 cfm/bushel for a bin
of wheat. The bin is 24 feet in diameter and the
wheat is 16 feet deep.
1. Calculate capacity of the bin.
pi x Diameter2
Area of bin floor = --------------
4
3.14 x 24 x 24
= -------------- = 452.16 ft2
4
Volume = Area x Depth
Volume = 452.16 x 16 = 7238 ft3
One Bushel = 1.244 ft3
Bushels = 7238 � 1.244 = 5818 bushels
2. Determine airflow required.
Bushels x cfm/bu = cfm
5818 x 1.0 = 5818 cfm
3. Determine velocity of air through grain.
cfm � bin floor area = cfm/sq. ft.
5818 x 452.16 = 12.9 cfm/sq. ft.
4. Determine resistance to airflow from Figure 14.
Approximately 0.32 inches of water per foot
of depth.
16 ft. grain depth
16 x .32 = 5.1 inches static pressure
Non-clean wheat static pressure =
5.1 x 1.3 = 6.6 inches
Add 0.5 inch for ducts and vents.
6.6 + 0.5 = 7.1 inches
5. Select the fan.
Air Horsepower =
Airflow Rate (cfm) x static pressure
------------------------------------
6320 x fan efficiency
Fan efficiency will vary from about 30 to 70%
over the fans operating static pressure range.
Drying fans operating at peak efficiency will
have an efficiency of about 65 percent.
5818 x 7.1
HP = --------------
6320 x 0.65
HP = 10.1
A centrifugal fan is needed due to the high
static pressure.
Of the fans in Table 10, a 10 hp low speed centrifugal
fan appears to have the ability to deliver the required
airflow against 7 inches of static pressure. If a fan
cannot be found to meet the requirements or a smaller
horsepower fan is desired, there is the option of using a
lower airflow rate or a shallower depth. Let's redo the
calculations with 12 feet of wheat in the bin.
Total bushels = 4364
Air flow rate required (cfm) = 4364 cfm
Velocity = 9.6 cfm/sq. ft.
Resistance = 0.29 inches of
water/foot x 12 x
1.3 + 5
= 5 inches static
pressure
A 5 hp low speed centrifugal fan as shown in Table 10
will deliver the required airflow against 5 inches static
pressure.
Table 10. Typical Fan Performance.*
-----------------------------------------------------------
Fan Static Pressure (Inches of Water)
-------------- ---------------------------------
Type Hp Dia. RPM 1 2 3 4 5
-----------------------------------------------------------
(in.) Airflow Rate (cfm)
Axial 3.0 18 3450 5700 4600 2650 1400
LS Cent. 3.0 24 1750 4580 4230 3820 3350 2550
HS Cent. 3.0 16 3500 2950 2550
IL Cent. 3.0 18 3450 3800 3600 3400 3000 2500
IL Cent. 3.0 24 3450 4100 4000 3750 3500 3250
Axial 5.0 24 3450 10500 9000 7000 4600 2900
LS Cent. 5.0 24 1750 7800 7000 6250 5550 4600
HS Cent. 5.0 13 3500 4350 3850
IL Cent. 5.0 24 3450 5500 5000 4400 4100 3900
Axial 7.5 24 3450 12500 11100 9450 6550 3900
LS Cent. 7.5 24 1750 10550 9750 8950 8000 7400
HS Cent. 7.5 15 3500 5700 5100
IL Cent. 7.5 28 3450 6200 6000 5700 5500 5200
Axial 10.0 26 3450 15500 14000 12250 9500 5800
LS Cent. 10.0 27 1750 13300 12400 11550 10500 9550
HS Cent. 10.0 18 3500 6800 6300
IL Cent. 10.0 28 3450 7700 7300 6800 6500 6300
-----------------------------------------------------------
-----------------------------------------------------------
Fan Static Pressure (Inches of Water)
-------------- ---------------------------------
Type Hp Dia. RPM 6 7 8 9 10
-----------------------------------------------------------
(in.) Airflow Rate (cfm)
Axial 3.0 18 3450
LS Cent. 3.0 24 1750
HS Cent. 3.0 16 3500 2120 1650 1000
IL Cent. 3.0 18 3450 1900
IL Cent. 3.0 24 3450 2650
Axial 5.0 24 3450
LS Cent. 5.0 24 1750 3300
HS Cent. 5.0 13 3500 3200 2200 1800
IL Cent. 5.0 24 3450 3600 2800 1800
Axial 7.5 24 3450
LS Cent. 7.5 24 1750 6100
HS Cent. 7.5 15 3500 4500 3800 2900
IL Cent. 7.5 28 3450 4800 4500 4000 3500 3000
Axial 10.0 26 3450 3400
LS Cent. 10.0 27 1750 8500 7300
HS Cent. 10.0 18 3500 5750 5100 4450
IL Cent. 10.0 28 3450 6000 5400 5100 4800 4400
-----------------------------------------------------------
LS = Low Speed Centrifugal Fan
HS = High Speed Centrifugal Fan
IL = In-Line Centrifugal Fan
*Consult a comparable table for the actual fan being selected.
Two or more fans are sometimes used to move air
through the grain. The fans can be attached either in
parallel or series. When the fans are attached in
parallel, each fan must be selected based on the total
static pressure. For example, one 5 hp low speed
centrifugal fan will move about 4600 cfm against a static
pressure of 5 inches. Two 5 hp low speed centrifugal fans
in parallel will each move about 3000 cfm against the
resulting higher static pressure of 6.5 inches. The total
airflow will be about 6000 cfm. Hooking fans in series
(tandem) allows developing twice the static pressure. For
example, a 5 hp axial fan will only move about 2900 cfm
against a 5.0 inch static pressure. That fan, however,
will move 8000 cfm at 2.5 inches of static pressure. Two
5 hp axial fans hooked in series will be able to move
about 8000 cfm against 5.0 inches of static pressure.
Remember that a 10 hp LSC fan will deliver about 9550
cfm at 5 inches of static pressure.
Caution: Be sure to use the manufacturer's data
for the specific fan you are using or plan to use. The
fan diameter, speed, horsepower and the static pressure
all affect the fan performance.
B
A C K | N E X T
Introduction
Types of Dryers and Drying
Heaters, Costs,
Safety and Managing Stored Grain
AE-701 (Revised), November 1994
NDSU Extension Service, North Dakota State University
of Agriculture and Applied Science, and U.S. Department
of Agriculture cooperating. Sharon D. Anderson, Director,
Fargo, North Dakota. Distributed in furtherance of the
Acts of Congress of May 8 and June 30, 1914. We offer our
programs and facilities to all persons regardless of
race, color, national origin, religion, sex, disability,
age, Vietnam era veterans status, or sexual orientation;
and are an equal opportunity employer.
This publication will be made in alternative
format upon request 701/231-7881.
North Dakota State University
NDSU Extension Service
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